Some IC devices include TFRs for their precision resistor needs. A TFR generally may have a thickness on the order of 0.1 μm or less, while a thick film resistor may be at least a thousand times thicker. Silicon Chromium (SiCr) and nickel chromium (NiCr) have been used for years as TFRs due to their relatively high electrical resistance in thin film form, relatively low temperature coefficient of resistance (TCR), and the ability to reliably carry relatively high current density. TFRs may be laser trimmed, particularly for precision ICs, such as for setting an operational-amplifier's input offset voltage or a voltage reference circuit's output voltage.
Laser trimming is accomplished by ablating away part of the TFR structure on the final IC die when in wafer form using a laser beam, such as using a neodymium-doped yttrium aluminum garnet (YAG) laser. The laser trimming is customized for each die, with the trimming generally controlled based on measured electrical data from the electrical testing of each IC die. As the TFR's effective cross-sectional area is reduced, its resistance increases. The laser trimming is commonly performed in conjunction with wafer probing and the laser beam may be transmitted through a passivation layer and through one or more Inter Level Dielectric (ILD) layers thereunder to reach the TFR(s).